Gas separation membranes comprising thin layers of amorphous homo- and copolymers of bis-2,2-trifluoromethyl-4,5-difluoro-1,3-dioxole coated onto porous substrates are disclosed in U.S. Pat. Nos. 5,053,059, 5,051,113 and 5,051,114, and in World Patent Application WO 9,015,662. Preferred copolymers contain about 65-99 mole % of dioxole and have a glass transition temperature of at least 140.degree. C. The membranes therefrom show oxygen/nitrogen selectivity of greater than 1.4:1. The fluoropolymer coatings were applied by casting from solution and are preferably less than 10 micrometers thick, more preferably less than 1 micrometer thick.
U.S. patent application Ser. No. 575,266, now U.S. Pat. No. 5,160,353, assigned to the same assignee as the instant case, describes a gas separation membrane comprising a relatively thick polymeric layer supported on a porous substrate, said polymeric layer being coated with an ultrathin layer of another polymer of relatively high selectivity; fluoropolymer coatings are not suggested. The ultrathin layers are about 0.01 to 0.2 micrometers thick.
Resnick, Polym. Prepr., Amer. Chem. Soc., Div. Polym. Chem., 31(1), 312 (1990) discloses amorphous fluoropolymers based on bis-2,2-trifluoromethyl-4,5-difluoro-1,3-dioxole.